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The complete guide to why your thermostat doesn't reach its set temperature

Thermostat problems

When the thermostat is set to 75 degrees and the HVAC system has been running for hours without reaching its set temperature, it wastes energy and money. People often assume that the problem is with the thermostat. While in many cases, it is the thermostat that is the cause of the issue, most of the time, other underlying problems cause the thermostat not to reach its set temperature.

It can get frustrating, and there are many reasons why a thermostat won’t reach its set temperature. This guide covers the top reasons why a thermostat won’t reach its set temperature from A to Z.

By the end of this guide, you can confidently know how to approach fixing a thermostat that doesn’t reach its set temperature, whether you solve the problem yourself or connect with an HVAC pro.

Jump to Section

  1. The thermostat is faulty

  2. The thermostat is in a bad location

  3. Heating or cooling system problems

  4. HVAC system airflow problems

  5. An over or undersized HVAC system

  6. Extreme outdoor temperatures

  7. Conclusion

The thermostat is faulty

A thermostat can become faulty because of defective internal components or wiring problems. An inner part can go bad over time because of improper wiring.

Improper wiring will cause a thermostat to stop working. A thermostat has terminals for wiring. These wires typically connect to the furnace or air handling unit, which gives the thermostat power and allows for the systems to communicate with each other.

When wired incorrectly, it can cause the internal components and electronics to short immediately or fail over time.

In most cases, if the thermostat has a problem with internal components, the thermostat will need to be replaced. Rarely do HVAC pros, and thermostat manufacturers repair thermostats with faulty internal components. Most thermostats have a warranty when purchased new, and if purchased and installed with an HVAC pro, it is typically extended.

In other cases, power surges can cause the internal components of a thermostat to short; here, too, the thermostat will need to be replaced.

The thermostat is in a bad location

When the thermostat is located poorly, it can not reach the set temperature. The thermostat reads the temperature from its set location. If the thermostat is located next to a window, exterior wall, or kitchen, it can cause the thermostat to show false readings relative to the rest of the home. If the thermostat is located where a register (vent) is blowing out the air, it can cause the thermostat to have false readings.

The ideal location for the thermostat is near the return grille (vent). Near the return grille, air flows throughout the home and into the HVAC system, where it is heated or cooled before circulating throughout the home again.

The air temperature by the return grille is the best location for a thermostat because as air circulates throughout the home, the temperatures balance to give a good average temperature reading before going back into the HVAC system for another cycle.

Moving the thermostat can be expensive because most thermostats are hard-wired to the HVAC system. Therefore moving it will require running wiring through the walls. One solution is to add a temperature sensor. Most smart thermostats allow you to connect a temperature sensor to the thermostat wirelessly, which can be placed in a specific room and moved accordingly or in a set location. When properly connected, the temperature is read from the sensor instead of the thermostat.

Heating or cooling problems

Heating or cooling problems can also be why the thermostat doesn’t reach the set temperature. Often, the thermostat shows that the system is working and the HVAC system is blowing air; however, it is not heating or cooling the home.

There are many reasons why an air conditioning, heat pump system, or gas furnace isn’t working correctly. Sometimes it's an easy fix, such as replacing the AC capacitor or unclogging an AC drain line. And other times, it is a more complex problem. You can learn more about common heating and cooling problems. Or you can connect with an HVAC pro to diagnose the problem.

One honest mistake often made is confusing the fan being on with the heating or cooling system working. So always be sure that the heating or cooling mode is on before concluding that the HVAC system isn’t working.

HVAC system airflow problems

Airflow problems are one of the most common reasons an HVAC system isn’t working correctly. Airflow problems can cause issues with an HVAC system's heating and cooling operation.

When an HVAC system doesn’t have proper airflow, it won’t be able to properly facilitate heat exchange from the evaporator coil or heat exchanger to the ambient air flowing through the HVAC system.

Airflow problems can cause the air conditioner to freeze, damage the compressor on air conditioners and heat pumps, and trigger a safety switch on gas furnaces—all of which will prevent the thermostat from reaching the set temperature.

A static pressure test is done to check for proper airflow through an HVAC system which should be performed and analyzed by an HVAC pro. Learn more about static pressure.

An over or undersized HVAC system

HVAC systems that are improperly sized can prevent a thermostat from reaching the set temperature. When a system is oversized, it can cause the system to cycle on and off too frequently. Which causes drastic temperature changes between on and off cycles and can cause discomfort in the home as the thermostat struggles to maintain a temperature range.

An undersized HVAC system will not have the heating or cooling capacity to cool or heat the whole home properly. And as a result, it will work harder and longer to try and achieve the desired temperature.

Both over and undersized systems will cause wear and tear and can cost you money in increased energy bills. Either because it turns on and off too frequently or is always on. Therefore, it is essential to size an HVAC system correctly.

In the event of undersized systems, it is possible to lower the home's heat load by improving insulation and infiltration rates. And in the event of an oversized system, it is possible to increase the heat load by adding ventilation systems.

To find out what size of an HVAC system you need, it is best to consult with an HVAC pro.

Extreme outdoor temperatures

Extreme outdoor temperatures can prevent the thermostat from reaching its set temperature. A properly designed HVAC system has an ideal range of operating temperatures, and each home has a unique heat load given its build quality and design. The heat load of a home determines the cooling and heating capacity needed. The greater the capacity, the faster it can cool or heat a space. And if the capacity is too small, it won't be able to reach the temperature at all.

In areas susceptible to extreme outdoor temperatures, the best practice is to ensure the home is adequately sealed and insulated to reduce the cooling or heating load during extreme temperatures. Not only will this reduce the heat load and help maintain comfortable living conditions, but it will also reduce energy consumption throughout the year.

Troubleshooting or replacing a thermostat

If you suspect the thermostat is not working because it is not reaching the set temperature, you can troubleshoot it or replace it. You can explore online resources for common thermostats such as the Google Nest or Ecobee. However, we recommend consulting with an HVAC pro. An HVAC pro will be able to properly diagnose the source of the problem and advise on how to fix it.

If you need to replace the thermostat, we recommend choosing a smart thermostat. Smart thermostats can save you a lot on energy costs because of built-in energy-saving features, plus some of them allow you to set schedules and monitor indoor air quality. They are usually also paired with a mobile app to enable you to control the thermostat from your phone conveniently.


We often assume that if the thermostat doesn’t reach its set temperature, there is a problem with the thermostat itself. As we discovered, a faulty thermostat is just one of the reasons why a thermostat doesn’t reach its set temperature. Most of the time, other problems cause a thermostat not to reach its set temperatures, such as thermostat location, heating, cooling, and airflow problems, HVAC system sizing, and extreme outdoor temperatures.

By now, you should have a good idea of why a thermostat doesn’t reach its set temperature, and you can follow some troubleshooting steps from the thermostat manufacturer. Or you may opt for replacing the thermostat and possibly adding a compatible temperature sensor.

Because there are so many reasons why a thermostat won’t reach its set temperature and the complexity of HVAC, we always recommend connecting with a pro to help diagnose and repair any problems.

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